Such energy systems have been known for many years. Major limitations of these systems have resulted from the practical application of what seems to be a simple direct chemical process. Hazardous materials, efficiencies, system size, plugging and clogging, gas formation, "plating out" or precipitation of the materials, membrane diffusion limitations, cost of materials and cost of operation highlight the practical problems. Another limitation of such systems is the loss of power output as the system discharges.
The fundamental chemical process in these systems is characterized by a chemical equation where the action proceeds in one direction in the charging of the system and in the opposite direction during the power generation by the system. An example of a redox system is given by the following chemical equation, the term "redox" defining reactions in which a reduction and a complementary oxidation occur together. EQU Cr.sup.2+ +Fe.sup.3+ .revreaction.Cr.sup.3+ +Fe.sup.2+ Eq. 1
In this system, limitations exist since the chromium is expensive and the chromium and iron, meant to be on either side of a membrane, cross over contaminating the other side. This necessitates frequent reprocessing of the electrolyte. Furthermore, noble metal catalysts are required to promote the reaction. Also, the system pH must be controlled to prevent gas formation.
U.S. Pat. No. 3,920,474 discloses a fuel cell which uses a source of sulfide ions as fuel in the negative chamber of a cell in which the positive and negative chambers are separated by an ion-transfer or mechanically porous membrane. The catholyte may comprise ferric chloride with the overall reaction being EQU 2Fe.sup.3+ +S.sup.2- .fwdarw.S+2Fe.sup.2+
This system acts as a fuel cell only for the delivery of power to a load and cannot be electrically recharged.
U.S. Pat. No. 4,485,154 discloses an electrically rechargeable anionically active energy storage and power delivery system using a sulfide-polysulfide anolyte reaction and an iodide-polyiodide, chloride-chlorine or bromide-bromine catholyte reaction.
An object of this invention is to provide a redox process for energy storage and power delivery process where no solids are deposited on the electrodes and where the electrolytes can be electrically restored at least initially.
It is yet another object of the invention to provide a safe process with a sufficient power density depending upon the application needs.
Another object of this invention is to provide substantially full power even at low states of charge of the system, i.e. the system power production stays constant over time maintaining substantially the same output until close to complete discharge.